Embodiments disclosed herein relate generally to printed circuit board (PCB) production and design, and more particularly to high-performance multi-layer PCB production and design.
Printed circuit boards, backplanes, midplanes, printed wiring boards, flex circuits, rigid flex-circuits, multi-chip-modules (MCM), and the like (herein referred to collectively as PCBs) have traditionally been used to provide an interconnect (a set of electrical circuit connections) for digital, analog and RF (radio frequency) signals that need to be routed between different locations within electronic devices including computers, networking systems and telecommunication equipment.
FIG. 1 is a cross-sectional view of a typical multilayer PCB 100 in accordance with the prior art. The PCB 100 includes multiple signal traces and/or planar conductive layers 110a-f separated by rigid or flexible planar insulating dielectric layers 115a-e bonded together. When routing an interconnect through a PCB, it often becomes necessary to move the interconnect between planar layers 110a-f. An electrical connection path used to accomplish this function is a via. Thru-hole vias 120 pass completely through the body of PCB 100, blind vias 125 only have one end of the via extend to the surface of the PCB 100, and buried vias 130 have neither end extend to the surface of the PCB 100.
Small conductive regions called pads may be directly attached to the via on one or more of the conductive layers. Pads that are a functional part of the interconnect are called functional pads 135a-f. Pads that are not needed in order to complete the interconnect as it is routed between layers are called non-functional pads 140. Pads may be electrically isolated from the rest of the layer by an non-conductive anti-pad region 145.
In FIG. 1, signal traces 150a and 150b on layers 110a and 110c respectively are electrically connected together through via 120 and functional pads 135a and 135b. Alternate interconnect connection schemes including press-fit connector pins 160 and discrete/planar passive components and circuits attached directly to a via pad can also be used in lieu of the signal trace 150a. 
The series portion of the via that is necessary in order to provide an electrical connection between the desired layers is called the via-thru section 155a-c. The via may also include a non-functional portion called a via-stub section 165a165b165cthat extends beyond the functional pads 135b 135d 135f. The end of the via-stub section that is not connected to the functional part of the interconnect 170a-c is the unterminated end of the via-stub section.
The presence of one or more via-stub sections in an interconnect can significantly reduce its useable bandwidth and detrimentally distort signals that propagate through the interconnect. One technique that has been used to mitigate the interconnect bandwidth and signal distortion limitations in the prior art includes back-drilling as shown in detail 180 of FIG. 1. A drill whose diameter 185 is wider than the finished hole-size is used to remove a substantial portion 190a of the via-stub section, leaving a short residual stub 190b that no longer has a significant impact on the interconnect's performance.
However, back-drilling is time consuming, expensive, and errors may irreparably damage the PCB 100. Further, back-drilling cannot be applied to via structures whose via-stub sections 165b-c do not extend all the way to the surface of the PCB unless provisions are taken to keep essential interconnects from being routed through the PCB regions 195a-b that will be removed during the backdrilling operation.